1. Field of the Invention
The present invention relates to naphthalenemethylenemalonic diesters and to UV (ultraviolet rays) absorbers and cosmetic compositions containing the diesters.
2. Description of the Related Art
Ultraviolet rays are divided into three groups: long wavelength ultraviolet rays (UV-A) having a wavelength from 400 to 320 nm, medium wavelength ultraviolet rays (UV-B) having a wavelength from 320 to 290 nm, and short wavelength ultraviolet rays (UV-C) having a wavelength shorter than 290 nm. UV-C is absorbed in the ozone layer and therefore scarcely reaches the surface of the earth, but UV-A and UV-B are known to cause various skin disorders. For example, if a human is exposed to UV-A, his skin is immediately melanized (instant melanizing action). Subsequently, the light energy reaches the corium to affect the elastic fibers of the walls of blood vessels and connective tissues. On the other hand, excessive exposure of the skin to UV-B causes onset of erythema or bulla, and accelerates melanin formation to cause disorders such as pigmentation. Moreover, it is considered that excessive exposure of UV-A and UV-B accelerates aging of the skin, causing spots, wrinkles, and freckles to eventually trigger the onset of skin cancers. Furthermore, ultraviolet rays are known to damage hair fibers by fading their color, roughening their texture, and causing a reduction in water absorption, loss of elasticity, and changes in the keratinous structure.
As the effects of ultraviolet rays on the human skin and hair fibers have become clear, compounds capable of absorbing UV-A and UV-B (ultraviolet rays absorbers) have been developed. For example, as UV-A absorbing agents, those containing derivatives of benzophenone or benzoylmethane are known. As UV-B absorbing agents, those containing derivatives of cinnamic acid, benzophenone, p-aminobenzoic acid, or salicylic acid are known. However, none of them meets all the following requirements (a) to (e) of UV absorbers: (a) it must absorb UV-A and UV-B as much as possible, (b) it must be stable against light and heat, (c) it must neither be toxic to the skin nor irritate the skin, or it must not give other harmful effects to the skin, (d) it must have a long-lasting effect, and (e) it must have excellent compatibility with a cosmetic base. Particularly, conventional UV absorbers have insufficient stabilities against light (UV rays); it is known that they are decomposed by UV rays, or reaction takes place when they are exposed to UV rays (see, for example, Int. J. Cosmetic Science, 10, 53 (1988)). Decomposition of UV absorbers results in a shortened term of effectiveness. In addition, the effects of the decomposition products themselves or products yielded from the reaction of decomposition products and other ingredients of the composition cannot be ignored (Fragrance Journal 84, 32, (1987)). Furthermore, some regions of UV-A and UV-B cannot be absorbed by conventional UV absorbers.
The present inventors synthesized numerous compounds having various naphthalene skeletal structures, and investigated their UV ray absorbing action, stability against light, etc. As a result, they found that the naphthalenemethylenemalonic diesters represented by the formula (1) described hereinlater have excellent ultraviolet ray absorbing action and excellent stability against light, and thus are useful as ingredients of cosmetic compositions. The present invention was completed based on this finding.